Discriminating the biophysical impacts of coastal upwelling and mud banks along the southwest coast of India

Abstract

Abstract Coastal upwelling and mud banks are two oceanographic processes concurrently operating along certain stretches of the southwest (Kerala) coast of India during the Southwest Monsoon period (June–September), facilitating significant enhancement in plankton biomass. Mud banks have scientific and societal attention from time immemorial, predominantly due to the large fisheries associated with them. In this paper, for the first time, the specific biophysical roles of these oceanographic processes have been discriminated, based on a focused 18 weekly/fortnightly time-series study (April to September 2014) in a mud bank-upwelling area (off Alappuzha, southwest coast of India). In conjunction with standard hydrographical and satellite remote sensing data, we utilised a FlowCAM to track the biophysical linkage in terms of plankton composition abundance and size structure at three locations (M1, M2 and M3) in the study area. During the Pre-Southwest Monsoon (April–May), the entire study area was warmer with low nitrate concentration in the surface waters, which caused lower biomass of autotrophs compared to the Southwest Monsoon (June–September). By the onset of the Southwest Monsoon (June), drastic hydrographical transformations took place in the study domain due to the Coastal upwelling, reflected as the surfacing of significantly cool, high nutrient and hypoxic waters. Concurrently, mud bank formed at location M2 due to the presence of relatively high-suspended sediments in the region, creating a localised calm environment conducive for fishing activities. In response to the hydrographical transformations in the entire study area during the Southwest Monsoon, the autotrophic plankton biomass and size structure experienced significant change. The micro-autotrophs biomass that was low during the Pre-Southwest Monsoon (av. 0.33 ± 0.2 mgC L− 1 at surface and av. 0.07 ± 0.04 mgC L− 1 at subsurface) noticeably increased during the Southwest Monsoon (av. 1.6 ± 0.4 mgC L− 1 at surface and av. 1.3 ± 0.2 mgC L− 1 at subsurface). The seasonal mean bio-volume of micro-autotrophs followed the same pattern in all three locations with the dominance of smaller individuals during the Pre-Southwest Monsoon (av. 55 ± 4.4 × 103 μm3 individual− 1 at surface and av. 67.1 ± 38.4 × 103 μm3 individual− 1 at subsurface). Relatively large phytoplankton dominated during the Southwest Monsoon (av. 77.3 ± 5.5 × 103 μm3 individual− 1 at surface and av. 90.3 ± 4.9 × 103 μm3 individual− 1 at subsurface). Similar spatial change in the plankton composition, biomass and size structure in all three locations suggested the dominant role of coastal upwelling, and not the mud bank, in shaping the dominant autotrophic plankton in the study domain. The nutrient enrichment in the region associated with the coastal upwelling facilitated the predominance of a classical short food chain capable of supporting large fish stocks. In addition to relatively calm mud bank, the hypoxic, cool upwelled waters that existed in the subsurface induce the fish stock to concentrate in the surface waters, making the relatively calm mud bank and adjoining areas a rich fishing ground for the native fishers.